Over the years, a large number of prior art disclosures have focused on the development of cable insulation wherein said cable is of a multiple layer design. For example, as early back as 1935-1940, a two layer insulating cable design was disclosed, the first layer made from natural rubber, and an outer flame retardant layer containing polycholoroprene or neoprene rubber. By the mid 1940's, first layers reportedly contained poly(ethylene) followed by an outer layer of poly(vinyl chloride). In the 1960's first layers were manufactured from ethylene-propylene diene terpolymer, or ethylene propylene copolymers, and the outer layer comprised chlorosulfontated polyethylene, neoprene, or Hypalon (chlorosulphonated polyethylene) rubber. In the mid 1960's first layers were manufactured from crosslinked polyethylene, and the outer layers of crosslinked poly(vinylidine fluoride).
For example, in U.S. Pat. No. 3,269,862 there is disclosed an electrical insulation material which comprises a first inner layer of a polyolefin and a second outer layer of poly(vinylidine fluoride) in which the polymer comprising each of the layers is crosslinked.
In U.S. Pat. No. 3,546,014, there is disclosed a method of manufacturing thin wall wire by first providing a insulation layer of chemically crosslinked polyethylene over a metal conductor. The surface of the insulation is etched over its circumference, and a flame retardant coating of a thermosetting halogenated polyolefin is applied uniformly over the polyethylene insulation.
In U.S. Pat. No. 4,051,298, there is disclosed the combination of a previously cured copolymer of ethylene and propylene adjoined to a subsequently cured elastomeric blend of ethylene and propylene mixed with chlorosulfonated polyethylene. The combination of materials is said to provide various advantages when used in electrically conducting wire and cable products while also providing an overlying strippable semiconductive layer.
In U.S. Pat. No. 4,184,001, there is disclosed an insulation system for electrical conductors, which comprises a layer of crosslinked flurocarbon polymer, selected from ethylene-tetrafluroethylene copolymer, ethylene chlorotrifluoroethylene copolymer and ethylene-tetrafluoroethylene terpolymer. This layer of polymer is then covered with a polyimide coating.
In U.S. Pat. No. 4,789,589 there is disclosed a conductor wire with an inner layer of insulation comprising cellular polyolefin and an outer layer of poly(vinyl chloride). The poly(vinyl chloride) is said to include a material compatible with the polyolefin (such as chlorinated PE), and is said to bond to the cellular polyolefin to hold the layers together.
In U.S. Pat. No. 5,059,483, there is disclosed shaped articles of crosslinked polymers comprising a first component having little or no crosslinking and high relative elongation, and a second component having a relatively high level of crosslinking and low elongation. The articles are described as being useful in the form of electrical insulation, the first component being adjacent to a wire or other conductor.
In U.S. Pat. No. 5,162,609, there is disclosed a fire resistant cable suitable for the transmission of high frequency signals which includes a core which contains a plurality of twisted pairs of insulated conductors and a jacket. The insulation system includes dual layers, the outer of which is a flame retardant plastic material. Specifically, the insulation system includes an inner layer of polyolefin plastic material and an outer layer of flame-retardant polyolefin plastic material characterized by a suitable low dissipation factor and dielectric constant. The outer layer of flame retardant polyethylene is about 0.003 inch.
U.S. Pat. No. 5,281,766 describes a motor lead wire that is overcoated with a primary insulation layer of polyolefin, protected by a jacket of poly(vinylidine fluoride) or a poly(vinylidine fluoride) copolymer having an approximate maximum thickness of 0.005 inches. The primary insulation is crosslinked and stabilized with a zinc salt of methylmercaptobenzimidazole and a hindered phenol anti-oxidant.
U.S. Pat. No. 5,358,786 describes a fire-resistant plenum-type electrical cable that is insulated with an inner layer of a flurocarbon copolymer and an outer layer of an abrasion resistant and flame resistant poly(vinyl chloride).
U.S. Pat. No. 5,514,837 describes a plenum cable having a plurality of insulated conductors enclosed by a jacket. In a preferred structure, one of the conductors is covered with an insulation layer of a flame retardant polyethylene or polypropylene resin, and one of the other conductors is insulated with fluorinated ethylene propylene (FEP).
As can be seen from the above review of the prior art, most of the multiple layer insulating cable designs reported to date, wherein the outer layer provides flame retardant characteristics, feature dissimilar material (resin) systems, which can contribute to incompatibility at the resin interfaces. In addition, none of the systems reported disclose multiple layer insulating design which can be crosslinked in a single crosslinking pass so that the multiple layers can be made to consistently stick together and strip together without significant separation. Furthermore, to date, a multiple layer insulating design of the aforementioned combined characteristics, that also provides product flexibility and a continuous service rating of greater than 10 kilovolts (kV) DC, or 5 kV AC such as is required in fine performing television set receiver cable, has not been available.
Accordingly, it is an object of this invention to overcome the disadvantages of the prior art and provide a multilayer insulating material, particularly suitable for high voltage wire insulation, wherein the layers are made compatible such that they will
It is also an objective of the present invention to manufacture such multilayer construction from at least two layers, wherein the layers as noted have crosslinking at the interface, and wherein the outer layer is also made flame retardant, with good flexibility, and wherein the inner layer is chosen for outstanding dielectric breakdown characteristics.
Finally, it is also an object of this invention to optimally crosslink such a two or more layer type insulation construction in a single crosslinking pass so that the layers not only adhere to one another, but strip together without noticeable separation, and wherein the crosslinked layers also minimize moisture penetration.